Diagnostic port

Does anyone have any snippets of info about the diagnostic port at all - protocols, codes or anything? I found an oscilliscope program on the web that works via the sound port on a PC. I connected it up to the diagnostic port on the GTA (with a little protective circuit in between) and managed to get a signal out that resembles serial data. So I'm thinking that it may be possible to write some software to interpret the signal, eventually ending up as a diagnostic program but that's as far as I got today. I will sit down and start to analyse the data to see if I can suss out what is going on but any useful info that exists out there would be a help.

as you have had no replys yet.......... the best i can offer is that there is a black cap on top of the port that you need to remove to access the info.